[Physics in daily life]
L.J.F. (Jo) Hermans - DOI: 10.1051/epn/2012201
ll
llLeiden University,e Netherlands - Hermans@Physics.LeidenUniv.nl
Boiling water
W
ater is a vital substance for any living
organism. But it is also an extraordinary substance. Take its boiling
point, for example. H2O has a much
higher boiling point than other light molecules. Indeed,
it is roughly a factor of five higher than for a simple diatomic molecule with a comparable mass like nitrogen.
It is also higher than that of polyatomic molecules like
CO2, SO2, CH4, and even higher than that of ordinary
alcohol, C2H5OH.
It is all due to the structure of the water molecule with its
large dipole moment. The attractive well depth is large,
and the molecules can escape from the well only at relatively high temperature. Hence the high boiling point.
But this also means that
the heat of vaporisation is
relatively large. This is fortunate, because it makes
water an efficient medium
to cool our body by evaporation when sweating.
Just how large is the heat
of vaporisation of water?
This is something we
know from experience
without probably realising
it. If we boil water it takes,
for example, 5 minutes to
reach the boiling point. If
we continue heating, it will take another 20 minutes or
so before the water has completely evaporated (which
is good, because it gives us time to save our kettle). This
means that evaporating water takes roughly four times
as much heat as raising its temperature from close to
0°C to 100°C. Since the latter takes about 420 kJ/kg,
the heat of vaporisation must be on the order of 2 MJ/
kg. Which it is.
Now an interesting problem arises when we boil water in
a pan which has a lid on it. Ask any layman this question:
What is the effect of having the lid on the pan: will it take
longer or shorter for the water to completely evaporate
compared to a pan without a lid? Or doesn’t it make any
difference? As we all know, when approaching boiling
point, much of the evaporating water will condense on
the lid and flow back into the liquid. So the answer will
probably be: Of course it takes longer when the lid is
on the pan.
Wrong. Do the experiment and see what happens. It
makes no noticeable difference. The explanation is easy.
Whether or not there is a lid on the pan, the heat supply
to the water remains the same. And where would the
heat go? Except for the (relatively small) heat losses by
radiation and conduction, all heat is used for evaporation. This means that there should be no difference, irrespective of the details of condensation and backflow
that occur under the lid.
If we are a little more precise, it is even the opposite of
what the layman expects. The reason is that radiation
losses are reduced by the metal lid with its emissivity
of perhaps 0.1, to be compared to almost 1 for the water surface. So there is even more net heat available to
vaporise water if the lid is on the pan, and evaporation
should even be a bit faster.
So, a simple kitchen experiment like boiling water
contains some nice physics. And to answer the seemingly complicated question of the influence of the lid
on the evaporation speed, all we need is the first law
of thermodynamics: energy conservation, that’s what
it boils down to. n
Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn/2012201
EPN 43/2 13